Add a drop safe async version of scope guard

[jsdt]

Description of Changes

This is a change to make sure our client cleanup is drop safe. Our previous code had:

    let mut client = scopeguard::guard((client, session), |(client, session)| {
        tokio::spawn(ws_client_teardown(client, session));
    });

    ws_client_actor_inner(&mut client.0, options, ws, sendrx).await;

    let (client, session) = ScopeGuard::into_inner(client);
    ws_client_teardown(client, session).await;

This has an race condition, because if the client task were cancelled immediately after ws_client_actor_inner(&mut client.0, options, ws, sendrx).await;, then we would stop running ws_client_teardown on the first deferred task. To make sure we always run that cleanup to completion, we need to run it inside a tokio::spawn. We could fix it with a spawn, but I added a helper that works like scopeguard::guard for async cleanup functions. I'm actually surprised that we don't seem to do this pattern anywhere else.

Rollback safety impact

None.

Expected complexity level and risk

Testing

This has some unit tests for the new guard.